This invention relates to heaters and more particularly to self-regulating heaters such as may be applied to the exterior of a refrigeration system compressor housing to maintain the lubricant therein above a predetermined temperature level.
In conventional refrigeration components, a refrigerant, such as those sold under the trademark "Freon" by E. I. du Pont de Nemours & Co., may, in liquid form, migrate from the condenser into the compressor lubricant. Upon start-up of the compressor, the sudden reduction in crankcase pressure may cause the refrigerant to boil, thus causing the lubricant to form with consequent loss of lubrication to other mechanical parts of the compressor. It has been conventional to employ a crankcase heater to maintain the compressor crankcase at a temperature above that of the rest of the refrigeration system so as to prevent the migration of refrigerant into the crankcase lubricant.
Formerly, fixed constant-resistance heaters were used for heating the crankcase. However, these heaters were not self-regulating and thus required further temperature controls to limit the heat output of the heater so as to prevent damage to the lubricating oil. These constant-resistance heaters and their associated temperature controls were complicated and expensive. Self-regulating sump heaters are disclosed in the coassigned U.S. Pat. Nos. 3,564,199 and 3,748,439. Briefly, these prior art self-regulating heaters employed a heater made of ceramic material having a positive temperature coefficient (PTC) of resistivity. Such heaters have a relatively low resistance at usual ambient temperatures, but after initial energization by a source of electrical power will self-heat and increase their temperature and resistance. Heat will be generated and the resistance will increase rapidly above a threshold or anomaly temperature until the heat generated balances the heat dissipated at which time the temperature and resistance stabilize with the resistance many times the initial value. Thus, these heaters are self-regulating at a temperature that will not exceed a safe value. Reference may be made to the above-mentioned prior art references for a more complete disclosure of the operating and physical characteristics of PTC heaters.
Many prior art self-regulating heaters utilized potting compounds to electrically insulate the PTC heater from the heater case, to provide increased heat transfer from the heater to the case and to locate the heater within the case. However, it has been found that certain potting compounds, such as epoxy resin materials or the gases emitted therefrom, may deleteriously affect the PTC heater (i.e., react with the PTC material and poison it) when it is operated at high temperatures.